Red Hat Enterprise Linux High Availability Add-On Overview User Manual
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Dokument
Annonce
Annonce
Red Hat Enterprise Linux 6
High Availability Add-On Overview
Overview of the High Availability Add-On for Red Hat Enterprise Linux
Edition 6
Red Hat Enterprise Linux 6 High Availability Add-On Overview
Overview of the High Availability Add-On for Red Hat Enterprise Linux
Edition 6
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Abstract
High Availability Add-On Overview pro vides an o verview o f the High Availability Add-On fo r Red
Hat Enterprise Linux 6 .
T able of Cont ent s
T able of Contents
.Int
. .roduct
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1. Do c ument Co nventio ns
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1.1. Typ o g rap hic Co nventio ns
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1.2. Pull-q uo te Co nventio ns
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1.3. No tes and Warning s
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2 . We Need Feed b ac k!
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1.1. Clus ter Bas ic s
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1.2. Hig h Availab ility Ad d -O n Intro d uc tio n
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1.3. Clus ter Infras truc ture
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2 .1. Clus ter Q uo rum
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2 .1.1. Q uo rum Dis ks
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2 .1.2. Tie-b reakers
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3 .1. Failo ver Do mains
3 .1.1. Behavio r Examp les
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3 .2. Servic e Po lic ies
3 .2.1. Start Po lic y
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3 .2.2. Rec o very Po lic y
3 .2.3. Res tart Po lic y Extens io ns
3 .3. Res o urc e Trees - Bas ic s / Definitio ns
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.3.1. Parent / Child Relatio ns hip s , Dep end enc ies , and Start O rd ering
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3 .4. Servic e O p eratio ns and States
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3 .4.1. Servic e O p eratio ns
3 .4.1.1. The freez e O p eratio n
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3 .4.1.1.1. Servic e Behavio rs when Fro z en
3 .4.2. Servic e States
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3 .5. Virtual Mac hine Behavio rs
3 .5.1. No rmal O p eratio ns
3 .5.2. Mig ratio n
3 .5.3. RG Manag er Virtual Mac hine Features
3 .5.3.1. Virtual Mac hine Trac king
3 .5.3.2. Trans ient Do main Sup p o rt
3 .5.3.2.1. Manag ement Features
.5.4. Unhand led Behavio rs
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3 .6 . Res o urc e Ac tio ns
3 .6 .1. Return Values
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5 .1. DLM Lo c king Mo d el
5 .2. Lo c k States
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6 .1. Clus ter Ad minis tratio n To o ls
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7 .1. VMs as Hig hly Availab le Res o urc es /Servic es
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7 .1.1. G eneral Rec o mmend atio ns
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
7 .1.1. G eneral Rec o mmend atio ns
7 .2. G ues t Clus ters
7 .2.1. Us ing fenc e_s c s i and iSCSI Shared Sto rag e
7 .2.2. G eneral Rec o mmend atio ns
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Revision
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2
Int roduct ion
Introduction
This document provides a high-level overview of the High Availability Add-On for Red Hat Enterprise
Linux 6.
Although the information in this document is an overview, you should have advanced working
knowledge of Red Hat Enterprise Linux and understand the concepts of server computing to gain a
good comprehension of the information.
For more information about using Red Hat Enterprise Linux, refer to the following resources:
Red Hat Enterprise Linux Installation Guide — Provides information regarding installation of Red Hat
Enterprise Linux 6.
Red Hat Enterprise Linux Deployment Guide — Provides information regarding the deployment,
configuration and administration of Red Hat Enterprise Linux 6.
For more information about this and related products for Red Hat Enterprise Linux 6, refer to the
following resources:
Configuring and Managing the High Availability Add-On Provides information about configuring and
managing the High Availability Add-On (also known as Red Hat Cluster) for Red Hat Enterprise
Linux 6.
Logical Volume Manager Administration — Provides a description of the Logical Volume Manager
(LVM), including information on running LVM in a clustered environment.
Global File System 2: Configuration and Administration — Provides information about installing,
configuring, and maintaining Red Hat GFS2 (Red Hat Global File System 2), which is included in
the Resilient Storage Add-On.
DM Multipath — Provides information about using the D evice-Mapper Multipath feature of Red Hat
Enterprise Linux 6.
Load Balancer Administration — Provides information on configuring high-performance systems
and services with the Red Hat Load Balancer Add-On (Formerly known as Linux Virtual Server
[LVS]).
Release Notes — Provides information about the current release of Red Hat products.
Note
For information on best practices for deploying and upgrading Red Hat Enterprise Linux
clusters using the High Availability Add-On and Red Hat Global File System 2 (GFS2) refer to
the article " Red Hat Enterprise Linux Cluster, High Availability, and GFS D eployment Best
Practices" on Red Hat Customer Portal at . https://access.redhat.com/kb/docs/D OC-40821.
This document and other Red Hat documents are available in HTML, PD F, and RPM versions on the
Red Hat Enterprise Linux D ocumentation CD and online at
http://access.redhat.com/documentation/docs.
1. Document Convent ions
This manual uses several conventions to highlight certain words and phrases and draw attention to
specific pieces of information.
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
1.1. T ypographic Convent ions
Four typographic conventions are used to call attention to specific words and phrases. These
conventions, and the circumstances they apply to, are as follows.
Mo no -spaced Bo l d
Used to highlight system input, including shell commands, file names and paths. Also used to
highlight keys and key combinations. For example:
To see the contents of the file my_next_bestsel l i ng _no vel in your current
working directory, enter the cat my_next_bestsel l i ng _no vel command at the
shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a key, all presented in mono-spaced bold and
all distinguishable thanks to context.
Key combinations can be distinguished from an individual key by the plus sign that connects each
part of a key combination. For example:
Press Enter to execute the command.
Press C trl +Al t+F2 to switch to a virtual terminal.
The first example highlights a particular key to press. The second example highlights a key
combination: a set of three keys pressed simultaneously.
If source code is discussed, class names, methods, functions, variable names and returned values
mentioned within a paragraph will be presented as above, in mo no -spaced bo l d . For example:
File-related classes include fi l esystem for file systems, fi l e for files, and d i r for
directories. Each class has its own associated set of permissions.
Pro p o rt io n al B o ld
This denotes words or phrases encountered on a system, including application names; dialog-box
text; labeled buttons; check-box and radio-button labels; menu titles and submenu titles. For
example:
Choose Syst em → Pref eren ces → Mo u se from the main menu bar to launch
Mo u se Pref eren ces. In the Butto ns tab, select the Left-hand ed mo use check
box and click C l o se to switch the primary mouse button from the left to the right
(making the mouse suitable for use in the left hand).
To insert a special character into a g ed it file, choose Ap p licat io n s →
Accesso ries → C h aract er Map from the main menu bar. Next, choose Search →
Fin d … from the C h aract er Map menu bar, type the name of the character in the
Search field and click Next. The character you sought will be highlighted in the
C haracter T abl e. D ouble-click this highlighted character to place it in the T ext
to co py field and then click the C o py button. Now switch back to your document
and choose Ed it → Past e from the g ed it menu bar.
The above text includes application names; system-wide menu names and items; application-specific
menu names; and buttons and text found within a GUI interface, all presented in proportional bold
and all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
4
Int roduct ion
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or
variable text. Italics denotes text you do not input literally or displayed text that changes depending
on circumstance. For example:
To connect to a remote machine using ssh, type ssh username@ domain.name at a
shell prompt. If the remote machine is exampl e. co m and your username on that
machine is john, type ssh jo hn@ exampl e. co m.
The mo unt -o remo unt file-system command remounts the named file system.
For example, to remount the /ho me file system, the command is mo unt -o remo unt
/ho me.
To see the version of a currently installed package, use the rpm -q package
command. It will return a result as follows: package-version-release.
Note the words in bold italics above: username, domain.name, file-system, package, version and
release. Each word is a placeholder, either for text you enter when issuing a command or for text
displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and
important term. For example:
Publican is a DocBook publishing system.
1.2. Pull-quot e Convent ions
Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mo no -spaced ro man and presented thus:
books
books_tests
Desktop
Desktop1
documentation drafts mss
downloads
images notes
photos
scripts
stuff
svgs
svn
Source-code listings are also set in mo no -spaced ro man but add syntax highlighting as follows:
static int kvm_vm_ioctl_deassign_device(struct kvm *kvm,
struct kvm_assigned_pci_dev *assigned_dev)
{
int r = 0;
struct kvm_assigned_dev_kernel *match;
mutex_lock(& kvm->lock);
match = kvm_find_assigned_dev(& kvm->arch.assigned_dev_head,
assigned_dev->assigned_dev_id);
if (!match) {
printk(KERN_INFO "%s: device hasn't been assigned
before, "
"so cannot be deassigned\n", __func__);
r = -EINVAL;
goto out;
}
kvm_deassign_device(kvm, match);
kvm_free_assigned_device(kvm, match);
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
o ut:
mutex_unlock(& kvm->lock);
return r;
}
1.3. Not es and Warnings
Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.
Note
Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should
have no negative consequences, but you might miss out on a trick that makes your life easier.
Important
Important boxes detail things that are easily missed: configuration changes that only apply to
the current session, or services that need restarting before an update will apply. Ignoring a
box labeled “ Important” will not cause data loss but may cause irritation and frustration.
Warning
Warnings should not be ignored. Ignoring warnings will most likely cause data loss.
2. We Need Feedback!
If you find a typographical error in this manual, or if you have thought of a way to make this manual
better, we would love to hear from you! Please submit a report in Bugzilla: http://bugzilla.redhat.com/
against the product R ed H at En t erp rise Lin u x 6 , the component doc-High_Availability_AddOn_Overview and version number: 6 . 6 .
If you have a suggestion for improving the documentation, try to be as specific as possible when
describing it. If you have found an error, please include the section number and some of the
surrounding text so we can find it easily.
6
Chapt er 1 . High Availabilit y Add- O n O verview
Chapter 1. High Availability Add-On Overview
The High Availability Add-On is a clustered system that provides reliability, scalability, and
availability to critical production services. The following sections provide a high-level description of
the components and functions of the High Availability Add-On:
Section 1.1, “ Cluster Basics”
Section 1.2, “ High Availability Add-On Introduction”
Section 1.3, “ Cluster Infrastructure”
1.1. Clust er Basics
A cluster is two or more computers (called nodes or members) that work together to perform a task.
There are four major types of clusters:
Storage
High availability
Load balancing
High performance
Storage clusters provide a consistent file system image across servers in a cluster, allowing the
servers to simultaneously read and write to a single shared file system. A storage cluster simplifies
storage administration by limiting the installation and patching of applications to one file system.
Also, with a cluster-wide file system, a storage cluster eliminates the need for redundant copies of
application data and simplifies backup and disaster recovery. The High Availability Add-On provides
storage clustering in conjunction with Red Hat GFS2 (part of the Resilient Storage Add-On).
High availability clusters provide highly available services by eliminating single points of failure and
by failing over services from one cluster node to another in case a node becomes inoperative.
Typically, services in a high availability cluster read and write data (via read-write mounted file
systems). Therefore, a high availability cluster must maintain data integrity as one cluster node takes
over control of a service from another cluster node. Node failures in a high availability cluster are not
visible from clients outside the cluster. (high availability clusters are sometimes referred to as failover
clusters.) The High Availability Add-On provides high availability clustering through its High
Availability Service Management component, rg manag er.
Load-balancing clusters dispatch network service requests to multiple cluster nodes to balance the
request load among the cluster nodes. Load balancing provides cost-effective scalability because
you can match the number of nodes according to load requirements. If a node in a load-balancing
cluster becomes inoperative, the load-balancing software detects the failure and redirects requests to
other cluster nodes. Node failures in a load-balancing cluster are not visible from clients outside the
cluster. Load balancing is available with the Load Balancer Add-On.
High-performance clusters use cluster nodes to perform concurrent calculations. A high-performance
cluster allows applications to work in parallel, therefore enhancing the performance of the
applications. (High performance clusters are also referred to as computational clusters or grid
computing.)
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Note
The cluster types summarized in the preceding text reflect basic configurations; your needs
might require a combination of the clusters described.
Additionally, the Red Hat Enterprise Linux High Availability Add-On contains support for
configuring and managing high availability servers only. It does not support high-performance
clusters.
1.2. High Availabilit y Add-On Int roduct ion
The High Availability Add-On is an integrated set of software components that can be deployed in a
variety of configurations to suit your needs for performance, high availability, load balancing,
scalability, file sharing, and economy.
The High Availability Add-On consists of the following major components:
Cluster infrastructure — Provides fundamental functions for nodes to work together as a cluster:
configuration-file management, membership management, lock management, and fencing.
High availability Service Management — Provides failover of services from one cluster node to
another in case a node becomes inoperative.
Cluster administration tools — Configuration and management tools for setting up, configuring,
and managing a the High Availability Add-On. The tools are for use with the Cluster Infrastructure
components, the high availability and Service Management components, and storage.
Note
Only single site clusters are fully supported at this time. Clusters spread across multiple
physical locations are not formally supported. For more details and to discuss multi-site
clusters, please speak to your Red Hat sales or support representative.
You can supplement the High Availability Add-On with the following components:
Red Hat GFS2 (Global File System 2) — Part of the Resilient Storage Add-On, this provides a
cluster file system for use with the High Availability Add-On. GFS2 allows multiple nodes to share
storage at a block level as if the storage were connected locally to each cluster node. GFS2
cluster file system requires a cluster infrastructure.
Cluster Logical Volume Manager (CLVM) — Part of the Resilient Storage Add-On, this provides
volume management of cluster storage. CLVM support also requires cluster infrastructure.
Load Balancer Add-On — Routing software that provides IP-Load-balancing. the Load Balancer
Add-On runs in a pair of redundant virtual servers that distributes client requests evenly to real
servers that are behind the virtual servers.
1.3. Clust er Infrast ruct ure
The High Availability Add-On cluster infrastructure provides the basic functions for a group of
computers (called nodes or members) to work together as a cluster. Once a cluster is formed using the
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Chapt er 1 . High Availabilit y Add- O n O verview
cluster infrastructure, you can use other components to suit your clustering needs (for example,
setting up a cluster for sharing files on a GFS2 file system or setting up service failover). The cluster
infrastructure performs the following functions:
Cluster management
Lock management
Fencing
Cluster configuration management
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Chapter 2. Cluster Management with CMAN
Cluster management manages cluster quorum and cluster membership. CMAN (an abbreviation for
cluster manager) performs cluster management in the High Availability Add-On for Red Hat Enterprise
Linux. CMAN is a distributed cluster manager and runs in each cluster node; cluster management is
distributed across all nodes in the cluster.
CMAN keeps track of membership by monitoring messages from other cluster nodes. When cluster
membership changes, the cluster manager notifies the other infrastructure components, which then
take appropriate action. If a cluster node does not transmit a message within a prescribed amount of
time, the cluster manager removes the node from the cluster and communicates to other cluster
infrastructure components that the node is not a member. Other cluster infrastructure components
determine what actions to take upon notification that node is no longer a cluster member. For
example, Fencing would disconnect the node that is no longer a member.
CMAN keeps track of cluster quorum by monitoring the count of cluster nodes. If more than half the
nodes are active, the cluster has quorum. If half the nodes (or fewer) are active, the cluster does not
have quorum, and all cluster activity is stopped. Cluster quorum prevents the occurrence of a " splitbrain" condition — a condition where two instances of the same cluster are running. A split-brain
condition would allow each cluster instance to access cluster resources without knowledge of the
other cluster instance, resulting in corrupted cluster integrity.
2.1. Clust er Quorum
Quorum is a voting algorithm used by CMAN.
A cluster can only function correctly if there is general agreement between the members regarding
their status. We say a cluster has quorum if a majority of nodes are alive, communicating, and agree
on the active cluster members. For example, in a thirteen-node cluster, quorum is only reached if
seven or more nodes are communicating. If the seventh node dies, the cluster loses quorum and can
no longer function.
A cluster must maintain quorum to prevent split-brain issues. If quorum was not enforced, quorum, a
communication error on that same thirteen-node cluster may cause a situation where six nodes are
operating on the shared storage, while another six nodes are also operating on it, independently.
Because of the communication error, the two partial-clusters would overwrite areas of the disk and
corrupt the file system. With quorum rules enforced, only one of the partial clusters can use the
shared storage, thus protecting data integrity.
Quorum doesn't prevent split-brain situations, but it does decide who is dominant and allowed to
function in the cluster. Should split-brain occur, quorum prevents more than one cluster group from
doing anything.
Quorum is determined by communication of messages among cluster nodes via Ethernet. Optionally,
quorum can be determined by a combination of communicating messages via Ethernet and through a
quorum disk. For quorum via Ethernet, quorum consists of a simple majority (50% of the nodes + 1
extra). When configuring a quorum disk, quorum consists of user-specified conditions.
Note
By default, each node has one quorum vote. Optionally, you can configure each node to have
more than one vote.
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Chapt er 2 . Clust er Management wit h CMAN
2.1.1. Quorum Disks
A quorum disk or partition is a section of a disk that's set up for use with components of the cluster
project. It has a couple of purposes. Again, I'll explain with an example.
Suppose you have nodes A and B, and node A fails to get several of cluster manager's " heartbeat"
packets from node B. Node A doesn't know why it hasn't received the packets, but there are several
possibilities: either node B has failed, the network switch or hub has failed, node A's network adapter
has failed, or maybe just because node B was just too busy to send the packet. That can happen if
your cluster is extremely large, your systems are extremely busy or your network is flakey.
Node A doesn't know which is the case, and it doesn't know whether the problem lies within itself or
with node B. This is especially problematic in a two-node cluster because both nodes, out of touch
with one another, can try to fence the other.
So before fencing a node, it would be nice to have another way to check if the other node is really
alive, even though we can't seem to contact it. A quorum disk gives you the ability to do just that.
Before fencing a node that's out of touch, the cluster software can check whether the node is still
alive based on whether it has written data to the quorum partition.
In the case of two-node systems, the quorum disk also acts as a tie-breaker. If a node has access to
the quorum disk and the network, that counts as two votes.
A node that has lost contact with the network or the quorum disk has lost a vote, and therefore may
safely be fenced.
Further information about configuring quorum disk parameters is provided in the chapters on Conga
and ccs administration in the Cluster Administration manual.
2.1.2. T ie-breakers
Tie-breakers are additional heuristics that allow a cluster partition to decide whether or not it is
quorate in the event of an even-split - prior to fencing. A typical tie-breaker construct is an IP tiebreaker, sometimes called a ping node.
With such a tie-breaker, nodes not only monitor each other, but also an upstream router that is on the
same path as cluster communications. If the two nodes lose contact with each other, the one that
wins is the one that can still ping the upstream router. Of course, there are cases, such as a switchloop, where it is possible for two nodes to see the upstream router - but not each other - causing what
is called a split brain. That is why, even when using tie-breakers, it is important to ensure that fencing
is configured correctly.
Other types of tie-breakers include where a shared partition, often called a quorum disk, provides
additional details. clumanager 1.2.x (Red Hat Cluster Suite 3) had a disk tie-breaker that allowed
operation if the network went down as long as both nodes were still communicating over the shared
partition.
More complex tie-breaker schemes exist, such as QD isk (part of linux-cluster). QD isk allows arbitrary
heuristics to be specified. These allow each node to determine its own fitness for participation in the
cluster. It is often used as a simple IP tie-breaker, however. See the qdisk(5) manual page for more
information.
CMAN has no internal tie-breakers for various reasons. However, tie-breakers can be implemented
using the API. This API allows quorum device registration and updating. For an example, look at the
QD isk source code.
You might need a tie-breaker if you:
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Have a two node configuration with the fence devices on a different network path than the path
used for cluster communication
Have a two node configuration where fencing is at the fabric level - especially for SCSI
reservations
However, if you have a correct network and fencing configuration in your cluster, a tie-breaker only
adds complexity, except in pathological cases.
12
Chapt er 3. RG Manager
Chapter 3. RGManager
RGManager manages and provides failover capabilities for collections of cluster resources called
services, resource groups, or resource trees. These resource groups are tree-structured, and have
parent-child dependency and inheritance relationships within each subtree.
How RGManager works is that it allows administrators to define, configure, and monitor cluster
services. In the event of a node failure, RGManager will relocate the clustered service to another node
with minimal service disruption. You can also restrict services to certain nodes, such as restricting
httpd to one group of nodes while mysq l can be restricted to a separate set of nodes.
There are various processes and agents that combine to make RGManager work. The following list
summarizes those areas.
Failover D omains - How the RGManager failover domain system works
Service Policies - Rgmanager's service startup and recovery policies
Resource Trees - How rgmanager's resource trees work, including start/stop orders and
inheritance
Service Operational Behaviors - How rgmanager's operations work and what states mean
Virtual Machine Behaviors - Special things to remember when running VMs in a rgmanager
cluster
ResourceActions - The agent actions RGManager uses and how to customize their behavior from
the cl uster. co nf file.
Event Scripting - If rgmanager's failover and recovery policies do not fit in your environment, you
can customize your own using this scripting subsystem.
3.1. Failover Domains
A failover domain is an ordered subset of members to which a service may be bound. Failover
domains, while useful for cluster customization, are not required for operation.
The following is a list of semantics governing the options as to how the different configuration
options affect the behavior of a failover domain.
preferred node or preferred member: The preferred node was the member designated to run a
given service if the member is online. We can emulate this behavior by specifying an unordered,
unrestricted failover domain of exactly one member.
restricted domain: Services bound to the domain may only run on cluster members which are also
members of the failover domain. If no members of the failover domain are available, the service is
placed in the stopped state. In a cluster with several members, using a restricted failover domain
can ease configuration of a cluster service (such as httpd), which requires identical configuration
on all members that run the service. Instead of setting up the entire cluster to run the cluster
service, you must set up only the members in the restricted failover domain that you associate with
the cluster service.
unrestricted domain: The default behavior, services bound to this domain may run on all cluster
members, but will run on a member of the domain whenever one is available. This means that if a
service is running outside of the domain and a member of the domain comes online, the service
will migrate to that member, unless nofailback is set.
ordered domain: The order specified in the configuration dictates the order of preference of
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members within the domain. The highest-ranking member of the domain will run the service
whenever it is online. This means that if member A has a higher-rank than member B, the service
will migrate to A if it was running on B if A transitions from offline to online.
unordered domain: The default behavior, members of the domain have no order of preference;
any member may run the service. Services will always migrate to members of their failover domain
whenever possible, however, in an unordered domain.
failback: Services on members of an ordered failover domain should fail back to the node that it
was originally running on before the node failed, which is useful for frequently failing nodes to
prevent frequent service shifts between the failing node and the failover node.
Ordering, restriction, and nofailback are flags and may be combined in almost any way (ie,
ordered+restricted, unordered+unrestricted, etc.). These combinations affect both where services start
after initial quorum formation and which cluster members will take over services in the event that the
service has failed.
3.1.1. Behavior Examples
Given a cluster comprised of this set of members: {A, B, C, D , E, F, G}.
O rd ered , rest rict ed f ailo ver d o main {A, B , C }
With nofailback unset: A service 'S' will always run on member 'A' whenever member 'A' is
online and there is a quorum. If all members of {A, B, C} are offline, the service will not run. If
the service is running on 'C' and 'A' transitions online, the service will migrate to 'A'.
With nofailback set: A service 'S' will run on the highest priority cluster member when a
quorum is formed. If all members of {A, B, C} are offline, the service will not run. If the service
is running on 'C' and 'A' transitions online, the service will remain on 'C' unless 'C' fails, at
which point it will fail over to 'A'.
U n o rd ered , rest rict ed f ailo ver d o main {A, B , C }
A service 'S' will only run if there is a quorum and at least one member of {A, B, C} is online.
If another member of the domain transitions online, the service does not relocate.
O rd ered , u n rest rict ed f ailo ver d o main {A, B , C }
With nofailback unset: A service 'S' will run whenever there is a quorum. If a member of the
failover domain is online, the service will run on the highest-priority member, otherwise a
member of the cluster will be chosen at random to run the service. That is, the service will
run on 'A' whenever 'A' is online, followed by 'B'.
With nofailback set: A service 'S' will run whenever there is a quorum. If a member of the
failover domain is online at quorum formation, the service will run on the highest-priority
member of the failover domain. That is, if 'B' is online (but 'A' is not), the service will run on
'B'. If, at some later point, 'A' joins the cluster, the service will not relocate to 'A'.
U n o rd ered , u n rest rict ed f ailo ver d o main {A, B , C }
This is also called a " Set of Preferred Members" . When one or more members of the failover
domain are online, the service will run on a nonspecific online member of the failover
domain. If another member of the failover domain transitions online, the service does not
relocate.
3.2. Service Policies
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Chapt er 3. RG Manager
RGManager has three service recovery policies which may be customized by the administrator on a
per-service basis.
Note
These policies also apply to virtual machine resources.
3.2.1. St art Policy
RGManager by default starts all services when rgmanager boots and a quorum is present. This
behavior may be altered by administrators.
autostart (default) - start the service when rgmanager boots and a quorum forms. If set to '0', the
cluster will not start the service and instead place it in to the disabled state.
3.2.2. Recovery Policy
The recovery policy is the default action rgmanager takes when a service fails on a particular node.
There are three available options, defined in the following list.
restart (default) - restart the service on the same node. If no other recovery policy is specified, this
recovery policy is used. If restarting fails, rgmanager falls back to relocate the service.
relocate - Try to start the service on other node(s) in the cluster. If no other nodes successfully
start the service, the service is then placed in the stopped state.
disable - D o nothing. Place the service in to the disabled state.
restart-disable - Attempt to restart the service, in place. Place the service in to the disabled state if
restarting fails.
3.2.3. Rest art Policy Ext ensions
When the restart recovery policy is used, you may additionally specify a maximum threshold for how
many restarts may occur on the same node in a given time. There are two parameters available for
services called max_restarts and restart_expire_time which control this.
The max_restarts parameter is an integer which specifies the maximum number of restarts before
giving up and relocating the service to another host in the cluster.
The restart_expire_time parameter tells rgmanager how long to remember a restart event.
The use of the two parameters together creates a sliding window for the number of tolerated restarts
in a given amount of time. For example:
<service name="myservice" max_restarts="3" restart_expire_time="300" ...>
...
</service>
The above service tolerance is 3 restarts in 5 minutes. On the fourth service failure in 300 seconds,
rgmanager will not restart the service and instead relocate the service to another available host in the
cluster.
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Note
You must specify both parameters together; the use of either parameter by itself is undefined.
3.3. Resource T rees - Basics / Definit ions
The following illustrates the structure of a resource tree, with a correpsonding list that defines each
area.
<service name="foo" ...>
<fs name="myfs" ...>
<script name="script_child"/>
</fs>
<ip address="10.1.1.2" .../>
</service>
Resource trees are XML representations of resources, their attributes, parent/child and sibling
relationships. The root of a resource tree is almost always a special type of resource called a
service. Resource tree, resource group, and service are usually used interchangeably on this wiki.
From rgmanager's perspective, a resource tree is an atomic unit. All components of a resource
tree are started on the same cluster node.
fs:myfs and ip:10.1.1.2 are siblings
fs:myfs is the parent of script:script_child
script:script_child is the child of fs:myfs
3.3.1. Parent / Child Relat ionships, Dependencies, and St art Ordering
The rules for parent/child relationships in the resource tree are fairly simple:
Parents are started before children
Children must all stop (cleanly) before a parent may be stopped
From these two, you could say that a child resource is dependent on its parent resource
In order for a resource to be considered in good health, all of its dependent children must also be
in good health
3.4 . Service Operat ions and St at es
The following operations apply to both services and virtual machines, except for the migrate
operation, which only works with virtual machines.
3.4 .1. Service Operat ions
The service operations are available commands a user may call to apply one of five available
actions, defined in the following list.
enable — start the service, optionally on a preferred target and optionally according to failover
domain rules. In absence of either, the local host where clusvcadm is run will start the service. If
16
Chapt er 3. RG Manager
the original start fails, the service behaves as though a relocate operation was requested (see
below). If the operation succeeds, the service is placed in the started state.
disable — stop the service and place into the disabled state. This is the only permissible
operation when a service is in the failed state.
relocate — move the service to another node. Optionally, the administrator may specify a preferred
node to receive the service, but the inability for the service to run on that host (e.g. if the service
fails to start or the host is offline) does not prevent relocation, and another node is chosen.
Rgmanager attempts to start the service on every permissible node in the cluster. If no permissible
target node in the cluster successfully starts the service, the relocation fails and the service is
attempted to be restarted on the original owner. If the original owner can not restart the service, the
service is placed in the stopped state.
stop — stop the service and place into the stopped state.
migrate — migrate the virtual machine to another node. The administrator must specify a target
node. D epending on the failure, a failure to migrate may result with the virtual machine in the
failed state or in the started state on the original owner.
3.4 .1 .1 . T he freeze Ope rat io n
RGManager can freeze services. D oing so allows users to upgrade rgmanager, CMAN, or any other
software on the system while minimizing down-time of rgmanager-managed services.
It also allows maintenance of parts of rgmanager services. For example, if you have a database and
a web server in a single rgmanager service, you may freeze the rgmanager service, stop the
database, perform maintenance, restart the database, and unfreeze the service.
3.4 .1.1.1. Service B eh avio rs wh en Fro z en
status checks are disabled
start operations are disabled
stop operations are disabled
Failover will not occur (even if you power off the service owner)
Important
Failure to follow these guidelines may result in resources being allocated on multiple hosts.
You must not stop all instances of rgmanager when a service is frozen unless you plan to
reboot the hosts prior to restarting rgmanager.
You must not unfreeze a service until the reported owner of the service rejoins the cluster
and restarts rgmanager.
3.4 .2. Service St at es
The following list defines the states of services managed by RGManager.
disabled — The service will remain in the disabled state until either an administrator re-enables
the service or the cluster loses quorum (at which point, the autostart parameter is evaluated). An
administrator may enable the service from this state.
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failed — The service is presumed dead. This state occurs whenever a resource's stop operation
fails. Administrator must verify that there are no allocated resources (mounted file systems, etc.)
prior to issuing a disable request. The only action which can take place from this state is disable.
stopped — When in the stopped state, the service will be evaluated for starting after the next
service or node transition. This is a very temporary measure. An administrator may disable or
enable the service from this state.
recovering — The cluster is trying to recover the service. An administrator may disable the service
to prevent recovery if desired.
started — If a service status check fails, recover it according to the service recovery policy. If the
host running the service fails, recover it following failover domain and exclusive service rules. An
administrator may relocate, stop, disable, and (with virtual machines) migrate the service from this
state.
Note
Other states, such as starti ng and sto ppi ng are special transitional states of the started
state.
3.5. Virt ual Machine Behaviors
RGManager handles virtual machines slightly differently from other non-VM services.
3.5.1. Normal Operat ions
VMs managed by rgmanager should only be administered using clusvcadm or another cluster aware
tool. Most of the behaviors are common with normal services. This includes:
Starting (enabling)
Stopping (disabling)
Status monitoring
Relocation
Recovery
To learn more about highly available virtual services, rever to Chapter 7, Virtualization and High
Availability.
3.5.2. Migrat ion
In addition to normal service operations, virtual machines support one behavior not supported by
other services: migration. Migration minimizes downtime of virtual machines by removing the
requirement for a start/stop in order to change the location of a virtual machine within a cluster.
There are two types of migration supported by rgmanager which are selected on a per-VM basis by
the migrate attribute:
18
Chapt er 3. RG Manager
live (default) — the virtual machine continues to run while most of its memory contents are copied
to the destination host. This minimizes the inaccessibility of the VM (typically well under 1 second)
at the expense of performance of the VM during the migration and total amount of time it takes for
the migration to complete.
pause - the virtual machine is frozen in memory while its memory contents are copied to the
destination host. This minimizes the amount of time it takes for a virtual machine migration to
complete.
Which migration style you use is dependent on availability and performance requirements. For
example, a live migration may mean 29 seconds of degraded performance and 1 second of complete
unavailability while a pause migration may mean 8 seconds of complete unavailability and no
otherwise degraded performance.
Important
A virtual machine may be a component of service, but doing this disables all forms of
migration and most of the below convenience features.
Additionally, the use of migration with KVM requires careful configuration of ssh.
3.5.3. RGManager Virt ual Machine Feat ures
The following section lists the various ways RGManager eases use of managing virtual machines.
3.5 .3.1 . Virt ual Machine T racking
Starting a virtual machine with cl usvcad m if the VM is already running will cause rgmanager to
search the cluster for the VM and mark the VM as started wherever it is found.
Administrators who accidentally migrate a VM between cluster nodes with non-cluster tools such as
vi rsh will cause rgmanager to search the cluster for the VM and mark the VM as started wherever it
is found.
Note
If the VM is running in multiple locations, RGManager does not warn you.
3.5 .3.2 . T ransie nt Do m ain Suppo rt
Rgmanager supports transient virtual machines which are supported by libvirt. This enables
rgmanager to create and remove virtual machines on the fly, helping reduce the possibility of
accidental double-starts of virtual machines due to the use of non-cluster tools.
Support of transient virtual machines also enables you to store libvirt XML description files on a
clustered file system so that you do not have to manually keep /etc/l i bvi rt/q emu in sync across
the cluster.
3.5.3.2.1. Man ag emen t Feat u res
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Adding or removing a VM from cluster.conf will not start or stop the VM; it will simply cause
rgmanager to start or stop paying attention to the VM
Failback (moving to a more preferred node) is performed using migration to minimize downtime.
3.5.4 . Unhandled Behaviors
The following conditions and user actions are not supported in RGManager.
Using a non-cluster-aware tool (such as virsh or xm) to manipulate a virtual machine's state or
configuration while the cluster is managing the virtual machine. Checking the virtual machine's
state is fine (e.g. virsh list, virsh dumpxml).
Migrating a cluster-managed VM to a non-cluster node or a node in the cluster which is not
running rgmanager. Rgmanager will restart the VM in the previous location, causing two
instances of the VM to be running, resulting in file system corruption.
3.6. Resource Act ions
RGManager expects the following return values from resource agents:
start - start the resource
stop - stop the resource
status - check the status of the resource
metadata - report the OCF RA XML metadata
3.6.1. Ret urn Values
OCF has a wide range of return codes for the monitor operation, but since rgmanager calls status, it
relies almost exclusively on SysV-style return codes.
0 - su ccess
stop after stop or stop when not running must return success
start after start or start when running must return success
n o n z ero - f ailu re
if the stop operation ever returns a nonzero value, the service enters the failed state and the
service must be recovered manually.
20
Chapt er 4 . Fencing
Chapter 4. Fencing
Fencing is the disconnection of a node from the cluster's shared storage. Fencing cuts off I/O from
shared storage, thus ensuring data integrity. The cluster infrastructure performs fencing through the
fence daemon, fenced .
When CMAN determines that a node has failed, it communicates to other cluster-infrastructure
components that the node has failed. fenced , when notified of the failure, fences the failed node.
Other cluster-infrastructure components determine what actions to take — that is, they perform any
recovery that needs to be done. For example, D LM and GFS2, when notified of a node failure,
suspend activity until they detect that fenced has completed fencing the failed node. Upon
confirmation that the failed node is fenced, D LM and GFS2 perform recovery. D LM releases locks of
the failed node; GFS2 recovers the journal of the failed node.
The fencing program determines from the cluster configuration file which fencing method to use. Two
key elements in the cluster configuration file define a fencing method: fencing agent and fencing
device. The fencing program makes a call to a fencing agent specified in the cluster configuration
file. The fencing agent, in turn, fences the node via a fencing device. When fencing is complete, the
fencing program notifies the cluster manager.
The High Availability Add-On provides a variety of fencing methods:
Power fencing — A fencing method that uses a power controller to power off an inoperable node.
storage fencing — A fencing method that disables the Fibre Channel port that connects storage to
an inoperable node.
Other fencing — Several other fencing methods that disable I/O or power of an inoperable node,
including IBM Bladecenters, PAP, D RAC/MC, HP ILO, IPMI, IBM RSA II, and others.
Figure 4.1, “ Power Fencing Example” shows an example of power fencing. In the example, the
fencing program in node A causes the power controller to power off node D . Figure 4.2, “ Storage
Fencing Example” shows an example of storage fencing. In the example, the fencing program in
node A causes the Fibre Channel switch to disable the port for node D , disconnecting node D from
storage.
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Fig u re 4 .1. Po wer Fen cin g Examp le
22
Chapt er 4 . Fencing
Fig u re 4 .2. St o rag e Fen cin g Examp le
Specifying a fencing method consists of editing a cluster configuration file to assign a fencingmethod name, the fencing agent, and the fencing device for each node in the cluster.
The way in which a fencing method is specified depends on if a node has either dual power supplies
or multiple paths to storage. If a node has dual power supplies, then the fencing method for the node
must specify at least two fencing devices — one fencing device for each power supply (refer to
Figure 4.3, “ Fencing a Node with D ual Power Supplies” ). Similarly, if a node has multiple paths to
Fibre Channel storage, then the fencing method for the node must specify one fencing device for
each path to Fibre Channel storage. For example, if a node has two paths to Fibre Channel storage,
the fencing method should specify two fencing devices — one for each path to Fibre Channel storage
(refer to Figure 4.4, “ Fencing a Node with D ual Fibre Channel Connections” ).
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Fig u re 4 .3. Fen cin g a N o d e wit h D u al Po wer Su p p lies
24
Chapt er 4 . Fencing
Fig u re 4 .4 . Fen cin g a N o d e wit h D u al Fib re C h an n el C o n n ect io n s
You can configure a node with one fencing method or multiple fencing methods. When you configure
a node for one fencing method, that is the only fencing method available for fencing that node. When
you configure a node for multiple fencing methods, the fencing methods are cascaded from one
fencing method to another according to the order of the fencing methods specified in the cluster
configuration file. If a node fails, it is fenced using the first fencing method specified in the cluster
configuration file for that node. If the first fencing method is not successful, the next fencing method
specified for that node is used. If none of the fencing methods is successful, then fencing starts again
with the first fencing method specified, and continues looping through the fencing methods in the
order specified in the cluster configuration file until the node has been fenced.
For detailed information on configuring fence devices, refer to the corresponding chapter in the
Cluster Administration manual.
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Chapter 5. Lock Management
Lock management is a common cluster-infrastructure service that provides a mechanism for other
cluster infrastructure components to synchronize their access to shared resources. In a Red Hat
cluster, D LM (D istributed Lock Manager) is the lock manager.
A lock manager is a traffic cop who controls access to resources in the cluster, such as access to a
GFS file system. You need it because without a lock manager, there would be no control over access
to your shared storage, and the nodes in the cluster would corrupt each other's data.
As implied in its name, D LM is a distributed lock manager and runs in each cluster node; lock
management is distributed across all nodes in the cluster. GFS2 and CLVM use locks from the lock
manager. GFS2 uses locks from the lock manager to synchronize access to file system metadata (on
shared storage). CLVM uses locks from the lock manager to synchronize updates to LVM volumes
and volume groups (also on shared storage). In addition, rg manag er uses D LM to synchronize
service states.
5.1. DLM Locking Model
The D LM locking model provides a rich set of locking modes and both synchronous and
asynchronous execution. An application acquires a lock on a lock resource. A one-to-many
relationship exists between lock resources and locks: a single lock resource can have multiple locks
associated with it.
A lock resource can correspond to an actual object, such as a file, a data structure, a database, or
an executable routine, but it does not have to correspond to one of these things. The object you
associate with a lock resource determines the granularity of the lock. For example, locking an entire
database is considered locking at coarse granularity. Locking each item in a database is considered
locking at a fine granularity.
The D LM locking model supports:
Six locking modes that increasingly restrict access to a resource
The promotion and demotion of locks through conversion
Synchronous completion of lock requests
Asynchronous completion
Global data through lock value blocks
The D LM provides its own mechanisms to support its locking features, such as inter-node
communication to manage lock traffic and recovery protocols to re-master locks after a node failure
or to migrate locks when a node joins the cluster. However, the D LM does not provide mechanisms to
actually manage the cluster itself. Therefore the D LM expects to operate in a cluster in conjunction
with another cluster infrastructure environment that provides the following minimum requirements:
The node is a part of a cluster.
All nodes agree on cluster membership and has quorum.
An IP address must communicate with the D LM on a node. Normally the D LM uses TCP/IP for
inter-node communications which restricts it to a single IP address per node (though this can be
made more redundant using the bonding driver). The D LM can be configured to use SCTP as its
inter-node transport which allows multiple IP addresses per node.
The D LM works with any cluster infrastructure environments that provide the minimum requirements
26
Chapt er 5. Lock Management
listed above. The choice of an open source or closed source environment is up to the user. However,
the D LM’s main limitation is the amount of testing performed with different environments.
5.2. Lock St at es
A lock state indicates the current status of a lock request. A lock is always in one of three states:
Granted — The lock request succeeded and attained the requested mode.
Converting — A client attempted to change the lock mode and the new mode is incompatible with
an existing lock.
Blocked — The request for a new lock could not be granted because conflicting locks exist.
A lock's state is determined by its requested mode and the modes of the other locks on the same
resource.
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Chapter 6. Configuration and Administration Tools
The cluster configuration file, /etc/cl uster/cl uster. co nf specifies the High Availability Add-On
configuration.The configuration file is an XML file that describes the following cluster characteristics:
Cluster name — Specifies the cluster name, cluster configuration file revision level, and basic
fence timing properties used when a node joins a cluster or is fenced from the cluster.
Cluster — Specifies each node of the cluster, specifying node name, node ID , number of quorum
votes, and fencing method for that node.
Fence D evice — Specifies fence devices in the cluster. Parameters vary according to the type of
fence device. For example for a power controller used as a fence device, the cluster configuration
defines the name of the power controller, its IP address, login, and password.
Managed Resources — Specifies resources required to create cluster services. Managed
resources includes the definition of failover domains, resources (for example an IP address), and
services. Together the managed resources define cluster services and failover behavior of the
cluster services.
The cluster configuration is automatically validated according to the cluster schema at
/usr/share/cl uster/cl uster. rng during startup time and when a configuration is reloaded.
Also, you can validate a cluster configuration any time by using the ccs_co nfi g _val i d ate
command.
An annotated schema is available for viewing at /usr/share/d o c/cmanX. Y . ZZ/cl uster_co nf. html (for example /usr/share/d o c/cman3. 0 . 12/cl uster_co nf. html ).
Configuration validation checks for the following basic errors:
XML validity — Checks that the configuration file is a valid XML file.
Configuration options — Checks to make sure that options (XML elements and attributes) are
valid.
Option values — Checks that the options contain valid data (limited).
6.1. Clust er Administ rat ion T ools
Managing Red Hat High Availability Add-On software consists of using configuration tools to specify
the relationship among the cluster components. The following cluster configuration tools are
available with Red Hat High Availability Add-On:
C o n g a — This is a comprehensive user interface for installing, configuring, and managing Red
Hat High Availability Add-On. Refer to Configuring and Managing the High Availability Add-On for
information about configuring and managing High Availability Add-On with C o n g a.
Luci — This is the application server that provides the user interface for Conga. It allows users
to manage cluster services and provides access to help and online documentation when
needed.
Ricci — This is a service daemon that manages distribution of the cluster configuration. Users
pass configuration details using the Luci interface, and the configuration is loaded in to
corosync for distribution to cluster nodes.
As of the Red Hat Enterprise Linux 6.1 release and later, the Red Hat High Availability Add-On
28
Chapt er 6 . Configurat ion and Administ rat ion T ools
provides support for the ccs cluster configuration command, which allows an administrator to
create, modify and view the cluster.conf cluster configuration file. Refer to the Cluster Administration
manual for information about configuring and managing the High Availability Add-On with the
ccs comand.
Note
system-co nfi g -cl uster is not available in RHEL 6.
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Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Chapter 7. Virtualization and High Availability
Various virtualization platforms are supported in conjunction with Red Hat Enterprise Linux 6 using
the High Availability and Resilient Storage Add-Ons. There are two supported use cases for
virtualization in conjunction with Red Hat Enterprise Linux High Availability Add-on.
This refers to RHEL Cluster/HA running on bare metal hosts that are themselves usable as
virtualization platforms. In this mode you can configure the cluster resource manager (rgmanager) to
manage virtual machines (guests) as highly available resources.
VMs as Highly Available Resources/Services
Guest Clusters
7.1. VMs as Highly Available Resources/Services
Both RHEL HA and RHEV provide mechanisms to provide HA virtual machines. Given the overlap in
functionality, care should be taken to chose the right product to fit your specific use case. Here are
some guidelines to consider when choosing between RHEL HA and RHEV for providing HA of VMs.
For Virtual machine and physical host counts:
If a large number of VMs are being made HA across a large number of physical hosts, using RHEV
may be the better solution as it has more sophisticated algorithms for managing VM placement
that take into consideration things like host CPU, memory and load information.
If a small number of VMs are being made HA across a small number of physical hosts, using
RHEL HA may be the better solution because less additional infrastructure is required. The
smallest RHEL HA VM solution needs two physical hosts for a 2 node cluster. The smallest RHEV
solution requires 4 nodes: 2 to provide HA for the RHEVM server and 2 to act as VM hosts.
There is no strict guideline for how many hosts or VMs would be considered a 'large number'. But
keep in mind that the maximum number of hosts in a single RHEL HA Cluster is 16, and that any
Cluster with 8 or more hosts will need an architecture review from Red Hat to determine
supportability.
Virtual machine usage:
If your HA VMs are providing services that are used are providing shared infrastructure, either
RHEL HA or RHEV can be used.
If you need to provide HA for a small set of critical services that are running inside of VMs, RHEL
HA or RHEV can be used.
If you are looking to provide infrastructure to allow rapid provisioning of VMs, RHEV should be
used.
RHEV VM HA is meant to be dynamic. Addition of new VMs to the RHEV 'cluster' can be done
easily and is fully supported.
RHEL VM HA is not meant to be a highly dynamic environment. A cluster with a fixed set of VMs
should be set up and then for the lifetime of the cluster it is not recommended to add or remove
additional VMs
RHEL HA should not be used to provide infrastructure for creating cloud-like environments due to
the static nature of cluster configuration as well as the relatively low physical node count
maximum (16 nodes)
30
Chapt er 7 . Virt ualiz at ion and High Availabilit y
RHEL 5 supports two virtualization platforms. Xen has been supported since RHEL 5.0 release. In
RHEL 5.4 KVM was introduced.
RHEL 6 only supports KVM as a virtualization platform.
RHEL 5 AP Cluster supports both KVM and Xen for use in running virtual machines that are managed
by the host cluster infrastructure.
RHEL 6 HA supports KVM for use in running virtual machines that are managed by the host cluster
infrastructure.
The following lists the deployment scenarios currently supported by Red Hat:
RHEL 5.0+ supports Xen in conjunction with RHEL AP Cluster
RHEL 5.4 introduced support for KVM virtual machines as managed resources in RHEL AP Cluster
as a Technology Preview.
RHEL 5.5+ elevates support for KVM virtual machines to be fully supported.
RHEL 6.0+ supports KVM virtual machines as highly available resources in the RHEL 6 High
Availability Add-On.
RHEL 6.0+ does not support Xen virtual machines with the RHEL 6 High Availability Add-On, since
RHEL 6 no longer supports Xen.
Note
For updated information and special notes regarding supported deployment scenarios, refer
to the following Red Hat Knowledgebase entry:
https://access.redhat.com/kb/docs/D OC-46375
The types of virtual machines that are run as managed resources does not matter. Any guest that is
supported by either Xen or KVM in RHEL can be used as a highly available guest. This includes
variants of RHEL (RHEL3, RHEL4, RHEL5) and several variants of Microsoft Windows. Check the
RHEL documentation to find the latest list of supported guest operating systems under each
hypervisor.
7.1.1. General Recommendat ions
In RHEL 5.3 and below, rgmanager utilized the native Xen interfaces for managing Xen domU's
(guests). In RHEL 5.4 this was changed to use libvirt for both the Xen and KVM hypervisors to
provide a consistent interface between both hypervisor types. In addition to this architecture
change there were numerous bug fixes released in RHEL 5.4 and 5.4.z, so it is advisable to
upgrade your host clusters to at least the latest RHEL 5.5 packages before configuring Xen
managed services.
For KVM managed services you must upgrade to RHEL 5.5 as this is the first version of RHEL
where this functionality is fully supported.
Always check the latest RHEL errata before deploying a Cluster to make sure that you have the
latest fixes for known issues or bugs.
Mixing hosts of different hypervisor types is not supported. The host cluster must either be all Xen
or all KVM based.
31
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
Host hardware should be provisioned such that they are capable of absorbing relocated guests
from multiple other failed hosts without causing a host to overcommit memory or severely
overcommit virtual CPUs. If enough failures occur to cause overcommit of either memory of virtual
CPUs this can lead to severe performance degradation and potentially cluster failure.
D irectly using the xm or libvirt tools (virsh, virt-manager) to manage (live migrate, stop. start)
virtual machines that are under rgmanager control is not supported or recommended since this
would bypass the cluster management stack.
Each VM name must be unique cluster wide, including local-only / non-cluster VMs. Libvirtd only
enforces unique names on a per-host basis. If you clone a VM by hand, you must change the
name in the clone's configuration file.
7.2. Guest Clust ers
This refers to RHEL Cluster/HA running inside of virtualized guests on a variety of virtualization
platforms. In this use-case RHEL Clustering/HA is primarily used to make the applications running
inside of the guests highly available. This use-case is similar to how RHEL Clustering/HA has always
been used in traditional bare-metal hosts. The difference is that Clustering runs inside of guests
instead.
The following is a list of virtualization platforms and the level of support currently available for
running guest clusters using RHEL Cluster/HA. In the below list, RHEL 6 Guests encompass both the
High Availability (core clustering) and Resilient Storage Add-Ons (GFS2, clvmd and cmirror).
RHEL 5.3+ Xen hosts fully supports running guest clusters where the guest operating systems are
also RHEL 5.3 or above:
Xen guest clusters can use either fence_xvm or fence_scsi for guest fencing.
Usage of fence_xvm/fence_xvmd requires a host cluster to be running to support fence_xvmd
and fence_xvm must be used as the guest fencing agent on all clustered guests.
Shared storage can be provided by either iSCSI or Xen shared block devices backed by either
host block storage or by file backed storage (raw images).
RHEL 5.5+ KVM hosts do not support running guest clusters.
RHEL 6.1+ KVM hosts support running guest clusters where the guest operating systems are either
RHEL 6.1+ or RHEL 5.6+. RHEL 4 guests are not supported.
Mixing bare metal cluster nodes with cluster nodes that are virtualized is permitted.
RHEL 5.6+ guest clusters can use either fence_xvm or fence_scsi for guest fencing.
RHEL 6.1+ guest clusters can use either fence_xvm (in the fence-vi rt package) or
fence_scsi for guest fencing.
The RHEL 6.1+ KVM Hosts must use fence_virtd if the guest cluster is using fence_virt or
fence_xvm as the fence agent. If the guest cluster is using fence_scsi then fence_virtd on the
hosts is not required.
fence_virtd can operate in three modes:
Standalone mode where the host to guest mapping is hard coded and live migration of
guests is not allowed
Using the Openais Checkpoint service to track live-migrations of clustered guests. This
requires a host cluster to be running.
32
Chapt er 7 . Virt ualiz at ion and High Availabilit y
Using the Qpid Management Framework (QMF) provided by the libvirt-qpid package. This
utilizes QMF to track guest migrations without requiring a full host cluster to be present.
Shared storage can be provided by either iSCSI or KVM shared block devices backed by either
host block storage or by file backed storage (raw images).
Red Hat Enterprise Virtualization Management (RHEV-M) versions 2.2+ and 3.0 currently support
RHEL 5.6+ and RHEL 6.1+ clustered guests.
Guest clusters must be homogeneous (either all RHEL 5.6+ guests or all RHEL 6.1+ guests).
Mixing bare metal cluster nodes with cluster nodes that are virtualized is permitted.
Fencing is provided by fence_scsi in RHEV-M 2.2+ and by both fence_scsi and fence_rhevm in
RHEV-M 3.0. Fencing is supported using fence_scsi as described below:
Use of fence_scsi with iSCSI storage is limited to iSCSI servers that support SCSI 3
Persistent Reservations with the preempt-and-abort command. Not all iSCSI servers
support this functionality. Check with your storage vendor to ensure that your server is
compliant with SCSI 3 Persistent Reservation support. Note that the iSCSI server shipped
with Red Hat Enterprise Linux does not presently support SCSI 3 Persistent Reservations,
so it is not suitable for use with fence_scsi.
VMware vSphere 4.1, VMware vCenter 4.1, VMware ESX and ESXi 4.1 supports running guest
clusters where the guest operating systems are RHEL 5.7+ or RHEL 6.2+. Version 5.0 of VMware
vSphere, vCenter, ESX and ESXi are also supported; however due to an incomplete WD SL
schema provided in the initial release of Vmware vSphere 5.0, the fence_vmware_soap utility does
not work on the default install. Refer to the Red Hat Knowledgebase
https://access.redhat.com/knowledge/ for updated procedures to fix this issue.
Guest clusters must be homogeneous (either all RHEL 5.7+ guests or all RHEL 6.1+ guests).
Mixing bare metal cluster nodes with cluster nodes that are virtualized is permitted.
The fence_vmware_soap agent requires the 3rd party VMware perl APIs. This software
package must be downloaded from VMware's web site and installed onto the RHEL clustered
guests.
Alternatively, fence_scsi can be used to provide fencing as described below.
Shared storage can be provided by either iSCSI or VMware raw shared block devices.
Usage of VMware ESX guest clusters is supported using either fence_vmware_so_ap or
fence_scsi.
Usage of Hyper-V guest clusters is unsupported at this time.
7.2.1. Using fence_scsi and iSCSI Shared St orage
In all of the above virtualization environments, fence_scsi and iSCSI storage can be used in place
of native shared storage and the native fence devices.
fence_scsi can be used to provide I/O fencing for shared storage provided over iSCSI if the iSCSI
target properly supports SCSI 3 persistent reservations and the preempt and abort command.
Check with your storage vendor to determine if your iSCSI solution supports the above
functionality.
33
Red Hat Ent erprise Linux 6 High Availabilit y Add- O n O verview
The iSCSI server software shipped with RHEL does not support SCSI 3 persistent reservations,
therefore it cannot be used with fence_scsi. It is suitable for use as a shared storage solution in
conjunction with other fence devices like fence_vmware or fence_rhevm, however.
If using fence_scsi on all guests, a host cluster is not required (in the RHEL 5 Xen/KVM and RHEL
6 KVM Host use cases)
If fence_scsi is used as the fence agent, all shared storage must be over iSCSI. Mixing of iSCSI
and native shared storage is not permitted.
7.2.2. General Recommendat ions
As stated above it is recommended to upgrade both the hosts and guests to the latest RHEL
packages before using virtualization capabilities, as there have been many enhancements and
bug fixes.
Mixing virtualization platforms (hypervisors) underneath guest clusters is not supported. All
underlying hosts must use the same virtualization technology.
It is not supported to run all guests in a guest cluster on a single physical host as this provides
no high availability in the event of a single host failure. This configuration can be used for
prototype or development purposes, however.
Best practices include the following:
It is not necessary to have a single host per guest, but this configuration does provide the
highest level of availability since a host failure only affects a single node in the cluster. If you
have a 2 to 1 mapping (two guests in a single cluster per physical host) this means a single
host failure results in two guest failures. Therefore it is advisable to get as close to a 1 to 1
mapping as possible.
Mixing multiple independent guest clusters on the same set of physical hosts is not supported
at this time when using the fence_xvm/fence_xvmd or fence_virt/fence_virtd fence agents.
Mixing multiple independent guest clusters on the same set of physical hosts will work if using
fence_scsi + iSCSI storage or if using fence_vmware + VMware (ESX/ESXi and vCenter).
Running non-clustered guests on the same set of physical hosts as a guest cluster is
supported, but since hosts will physically fence each other if a host cluster is configured, these
other guests will also be terminated during a host fencing operation.
Host hardware should be provisioned such that memory or virtual CPU overcommit is avoided.
Overcommitting memory or virtual CPU will result in performance degradation. If the
performance degradation becomes critical the cluster heartbeat could be affected which may
result in cluster failure.
34
Revision Hist ory
Revision History
R evisio n 1- 13
Wed O ct 8 2014
GA Release for Red Hat Enterprise Linux 6.6
St even Levin e
R evisio n 1- 12
T h u Au g 7 2014
Beta Release for Red Hat Enterprise Linux 6.6
St even Levin e
R evisio n 1- 11
Resolves: #852720
Small editorial issues
Fri Au g 1 2014
St even Levin e
R evisio n 1- 10
Fri Ju n 6 2014
D raft for Red Hat Enterprise Linux 6.6
St even Levin e
R evisio n 1- 7
Wed N o v 20 2013
Release for GA of Red Hat Enterprise Linux 6.5
Jo h n H a
R evisio n 1- 4
Mo n Feb 18 2013
Release for GA of Red Hat Enterprise Linux 6.4
Jo h n H a
R evisio n 1- 3
Mo n Ju n 18 2012
Release for GA of Red Hat Enterprise Linux 6.3
Jo h n H a
R evisio n 1- 2
Update for 6.2 release
Fri Au g 26 2011
Jo h n H a
R evisio n 1- 1
Initial Release
Wed N o v 10 2010
Pau l K en n ed y
35
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